1. Field of the Invention
This invention relates to thermally printable imaging materials containing color-forming precursors encapsulated in photosensitive microcapsules.
2. Description of Related Art
Photohardenable imaging systems employing microencapsulated radiation sensitive compositions are the subject of U.S. Pat. Nos. 4,399,209, 4,416,966 and 4,440,846. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photohardenable composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photohardenable composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color former. Exposure to actinic radiation hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is subjected to a uniform rupturing force by passing the sheet through the nip between a pair of pressure rollers.
U.S. Pat. No. 4,440,846 discloses a self-contained imaging sheet in which the encapsulated color former and the developer material are co-deposited on one surface of a single substrate as one layer or as two contiguous layers. Upon image-wise exposing the imaging sheet and passing the sheet through pressure rollers, the microcapsules rupture and image-wise release the internal phase whereupon the color former migrates to the developer material where it reacts with the developer material and forms a colored image.
Variations of the processes described above have also been described for making panchromatic full color images, as described in U.S. Pat. Nos. 4,772,530 and 4,772,541. In the color system, photosensitive microcapsules containing cyan, magenta and yellow color-forming dyes are layered on a support and exposed to red, green and blue light. Capsules containing the cyan precursor are photohardened by exposure to red light; capsules containing the magenta precursor are photohardened by exposure to green light and capsules containing the yellow precursor are photohardened by exposure to blue light. Selective exposure of the capsule layer to light of different wavelength, followed by rapture of non-exposed capsules gives rise to full color images on the support sheet after contact of the dye contents of the capsule with a suitable developer.
U.S. Pat. No. 5,783,353 further discloses a self contained imaging assembly capable of producing full color images as described above wherein a composition comprising photohardenable microcapsules and a developer material is disposed between a first transparent support and a second support which may be opaque or transparent to form a sealed assembly. The assembly is image-wise exposed to actinic radiation and subjected to a uniform rupturing force to provide an image in said composition which is visible against the second opaque support when viewed through the first transparent support or, if the second support is transparent, when the image is viewed as a transparency.
One major drawback associated with the systems described above is that there is no mechanism for fixing the image after the capsules are fractured (opened) and the dye precursor contacts the developer. The image contrast thus becomes sharper and sharper after printing which leads to poor preservation of mid-tone images. Also, the process has relatively low printing speed, poor environmental stability and reproducibility, relatively poor resolution as a function of larger microcapsule size (up to 15 microns) required for fracturing and requires the use of bulky capsule fracturing equipment such as described in U.S. Pat. No. 5,550,627.
Thermal techniques for producing images involving the use of encapsulated color precursors are also known. U.S. Pat. No. 4,598,035 discloses heat sensitive recording material comprising a support having on a surface an imaging layer comprising a plurality of microcapsules encapsulating a color precursor disposed in a binder containing a color developer or coupler. Selective image-wise heating of the layer by proximate contact with a thermal recording head causes the color precursor and/or the developer to permeate through the microcapsule walls selectively only in the heated areas, resulting in a colored image.
For color applications, a support comprising three separate imaging layers containing encapsulated diazonium salts or leuco dyes and non-encapsulated coupler or developer dispersions capable of forming yellow, magenta and cyan images when the coupler or developer is contacted with the diazonium salt or leuco dye are provided. The encapsulant materials in each layer are designed to be sensitive to different levels of heat energy such that image-wise thermal head exposure at low heat selectively activates the topmost yellow layer, image-wise thermal head exposure at moderate heat selectively activates the middle magenta layer and finally exposure at higher heat activates the bottom cyan layer. The structure is exposed to ultra violet light after heat exposure in each of the first two steps to decompose the diazonium salt which fixes the yellow and magenta images. Multicolor images of this type are disclosed in U.S. Pat. No. 5,168,029.
Thermal processes of the type described above require complex imaging materials and are subject to printing/temperature variations which can lead to poor image quality and image reproducibility. Also, the need to precisely vary and modulate the temperatures and thermal energy in the printing head for image-wise thermal heating of the heat exposed layers necessarily slows down the process.
In accordance with this invention, a heat sensitive recording sheet is provided comprising a support layer comprising transparent sheet material and a heat sensitive imaging layer disposed on one surface of said first support layer, said imaging layer comprising a mixture of:
i) a plurality of light sensitive microcapsules comprising a polymeric matrix or wall material and a photo-polymerizable or photocrosslinkable composition, a photoinitiator and a color dye precursor;
ii) a finely divided particulate developer or coupler material having a melting point or melt flow temperature in excess of 70xc2x0 C. which is reactive in contact with said color precursor to form a color;
said microcapsules characterized as permeable to said developer material after said recording sheet is heated to a temperature above the melting point or melt flow temperature of said developer or coupler, preferably to a temperature also above the Tg of the polymeric matrix or wall material, sufficient to form an image in an imaging process and impermeable to said developer or coupler material after said microcapsules are photohardened by exposure to a pattern of actinic radiation and said recording sheet is heated to a temperature above the melting point or melt flow temperature of said developer or coupler in an imaging process. The invention also provides a thermal imaging process for forming mono or polychromatic images comprising:
a) providing a heat sensitive recording sheet comprising:
i) a first support layer comprising transparent sheet material;
ii) a thermal slip layer disposed on one surface of said first support layer;
iii) a heat sensitive imaging layer disposed on the opposite surface of said first support layer; and
iv) a second opaque or transparent sheet material bonded to said heat sensitive imaging layer,
said imaging layer comprising a mixture of (1): a plurality of light sensitive microcapsules comprising a polymeric wall or matrix material and a photo-polymerizable or photocrosslinkable composition, a photoinitiator and a color dye precursor as an internal phase, and (2) a finely divided particulate developer or coupler material having a melting point or melt flow temperature in excess of 70xc2x0 C. which is reactive in contact with said color precursor to form a color;
b) image-wise exposing said recording sheet to a pattern of actinic radiation wherein microcapsules sensitive to said radiation are selectively photohardened; and
c) heating said recording sheet to a temperature above the melting point or melt flow temperature of said developer or coupler material such that the developer or coupler material flows into contact with said microcapsules, wherein photohardened microcapsules are impermeable to said developer or coupler material and non-photohardened microcapsules are permeable to said developer or coupler material sufficient to form a color image.
The heat sensitive recording sheets of the invention may be characterized as a self contained imaging assembly capable of producing color images in two simple steps: 1) image-wise exposure of the heat sensitive recording sheet to actinic radiation, followed by 2) uniformly and rapidly heating the exposed heat sensitive recording sheet under low pressure to a temperature well above the melting point or melt flow temperature of the developer or coupler, such as by contact with a thermal printing head, sufficiently to develop an image. The process is considerably simpler and faster than the prior art imaging processes described above and does not require the use of high pressure roller or other microcapsule fracturing equipment, registration mechanism for multiple pass thermal printing, narrow band ultra-violet light sources or multiple image layers. Images produced by the system of the invention are of high density, excellent resolution and have good mid-tone image preservation.
The image assembly of the present invention may be exposed in any suitable photo exposure device to provide an image. The imaging assembly of this invention is especially suitable for exposure using a liquid crystal array or light emitting diodes driven by a computer generated signal or a video signal for the reproduction of images from a video cassette recorder, a camcorder, or the like.